The rumbling of the Land Rover engine died when she twisted the key. Cassia Ross glanced out at the ominous expanse of grey overhead, took in a breath of the musty, stuffy air trapped in the car, and she popped the door.

Gravel crunched under her boots. When she rounded the vehicle, a blast of chilly wind numbed her cheeks and tossed her plait off her shoulder, so she zipped up her coat to keep the breeze out. Her backpack was in the boot. She grabbed it before locking the car and heading to the Dale of Walls beach trail.

The sea air was fresh and light with a hint of salt. It was the sort of breeze that blew away mustiness, and after a morning of helping in the airless dairy, it was exactly what Cass needed. She gulped it down into her lungs while she walked down to where the strip of grey shingle was nestled in the bay. After the long morning, and with a call with her little brother scheduled for the evening, connection permitting, it was a moment of respite.

Even for Shetland, the Dale of Walls beach was pretty isolated, far out on the western Mainland. The beaches on the eastern shore were sandy and filled with tourists, but Cass found the Dale of Walls to be an untouched, unknown paradise. Sure, the locals popped down with their dogs to look at the bay, to see the Atlantic stretch off into the distance, steely grey and glimmering, not reaching land again until the shores of Greenland, but that was all. The farm was so busy, she had so much work and reading to do (she'd never thought in her life that she'd find herself writing reflective cycles about milking), and it was a welcome escape. She popped down here often with a flask of tea and a novel, and today was no different.

The dull stretch of shingle came into view, lining the base of a small bay, and the cold wind redoubled. Her hair blew out behind her. Just coming here, out of range of her phone reception, away from the farm, she found the rugged remoteness settled her heartbeat, and the wind blew in her ears so loudly that she couldn't think too hard.

She swung the backpack from her shoulder to retrieve a blanket. After a couple of hours in the fresh air and a few chapters of her Kathy Reichs, a gift from her mum, she'd be ready for a call with her brother. She wasn't in the business of letting Lee down anymore.

An odd shape on the shoreline caught her eye. A dark-haired person huddled over something. Some sort of bag, maybe? But no – when she crashed closer through the shingle, her backpack clasped in one hand, she could see it was a young woman and a dog. Maybe a husky? Both looked soaked to the skin.

Cass frowned. Autumn was in the air here in Shetland, and the waters were best avoided for swimming at the best of times. That might have been the southerner in her talking, but highs of 13°C didn't count as warm to her. Either way, the breeze was cold, persistent, and that could easily mean hypothermia.

She paused for a moment, letting her backpack dangle just above the shingle. There was something about the scene that didn't sit right. The dog was laid on the shingle, unmoving, and it looked like there was a patterned blanket draped over its back. Cass looked a little closer, and her heart dropped into her stomach. The scarlet patches looked too bright, too irregular, to be any sort of fabric pattern.

Was that blood?

Cass threw her backpack back on and took off across the beach. Pebbles crashed and slid under her feet. She'd done the basics of dog anatomy back in the previous September, but she hadn't touched on trauma yet, and wouldn't until her third year. Even if she knew more than some basic suturing techniques, it wasn't like she carried the inside of a vet clinic around with her. What was she going to suture with, seaweed and fish bone?

A cold breeze numbed her cheeks when she descended the beach. "What happened? Are you okay?" She was close enough that the wind couldn't snatch her words away, and the girl looked up. Close up, Cass was struck by how young she looked. Dark sodden hair hung down her face like lank seaweed. Her clothes were drenched, but more than that, they were tattered and worn.

What was she doing here?

"No, I – we need help, I – I can't explain –" The girl's voice wavered on the wind. She had a Yorkshire accent that made Cass raise an eyebrow. Unusual for Shetland. She drew a little nearer, and then her gaze landed on the dog.

Her mouth fell open in horror.

What she'd thought was a blanket was nothing of the sort. Either side of the dog's back were draped what were unmistakably seagull wings, probably a herring gull by the size of them, that were starting to curl and stiffen with rigor mortis. The grey feathers fluttered in the breeze. They were sewn into the fur at the dog's spine with ugly black thread, and blood leaked out onto the white fur. Where the two met there was exposed muscle and pulsing blood vessels. Cass dragged her gaze away and swallowed hard against the nausea boiling in her stomach. Who would do something like this?

"What the –?" She peered at the dog's face. Its eyelids fluttered, small whimpers squeaking.

Poor thing.

"I – I can't explain, it's too crazy – just – please help..."

Cass glanced up at the girl. Now she was looking, there was something familiar about the shape of her face, and when she raised her head, the curtains of hair parting, it all clicked into place. She knew a girl with a scar just like that, cleaving down through her eye and twisting her mouth, though she'd only ever seen her from a distance. A girl who had a white pet malamute. A girl with a gunman on her tail.

Eliza Whittle.

The thought froze her for a second. Last time she'd heard anything about her it had been when she'd left her neighbour's house to go give a statement at the police station, and then she'd disappeared into thin air like a puff of smoke. She'd dragged Lee into her problems and gotten him caught in the literal crossfire. That was in London, over 700 miles away. What on earth was she doing in Shetland?

"Please?"

Eliza's whimper snatched her back to reality. It didn't matter who she was, or how she'd got here. Not until the poor dog between them was being looked after, anyway. Then there would be plenty of time for questions.

Cass wrenched her blanket out of her backpack and dumped it onto the shingle. "Here, we'll lift her with this and take her to my car. I know the local vet – he'll help, I'm sure." She looked back at Eliza, thinner than she remembered and shivering in her soaked clothes. "You sure you can lift?"

The girl's jaw set. "Yeah." She gave a brusque nod.

Between the two of them they rolled the dog into the blanket, blood soaking into the abstract patterns. Cass winced at every whimper and whine. "On three, and then we'll lift, okay?" All she could see of Eliza was her sodden hair, once again curtaining her face, but she thought she caught a nod. "One, two, three." With a great heave, they lifted the dog up between them. They headed up the beach.

The shingle didn't make it an easy journey. Their feet sunk and slid on the unsteady slope, the cold breeze biting at exposed fingers, but eventually the two of them stumbled back to the Land Rover. Cass fumbled one-handedly for her keys, and the two of them bundled the dog into the back seat. It was still warm and whimpering. At least that meant it was alive.

"Get in the front, alright? It's about a ten-minute drive, it won't be long." She hurried around the car and swung herself into the driver's seat. Another twist of the key and the engine roared into life just as Eliza climbed up to the passenger side.

"Okay – thanks." The girl dripped beside her while she clicked her seatbelt into place.

"S'okay." Cass shoved her foot on the accelerator. They bounced backwards onto the gravel road and into the quickest three-point turn of her life.

Her mind was already a hundred metres down the road, planning the quickest way to the vet's place without having to reroute via the farm, but Eliza's fear was palpable in the air. "It's all gonna be fine, Eliza. Jim's really good at his job – I've seen him get a cow off a fence post and get her all stitched up just fine before. I'm sure this is something he can handle, though I guess seagull wings on a malamute aren't exactly what you see every day..." She trailed off, suddenly aware of the frost inside the cab.

Eliza broke the silence with a sharp snap. "How the fuck d'you know who I am?"